OK, problem solved. Here's my story:
I followed every step mentioned in previous discussion to change languages, but the Chinese characters that I keyed in still end up like "????" although the menus and interface showing correct Chinese. So that means it's not the language pack's issue, and I have no problems to show Chinese with Joomla website either, then it should not be Joomla itself.
So I guess it must be something wrong with the character set. since other people in this board do not have my problem and I followed steps as well, then it should not be the page files. the only thing left is database. Because I'm a newbie to Joomla and VM, when I finished install VM, I chose to install sample data, trying to get an rough idea what VM look like, so VM installed sample data automatically into the database with "latin1_swedish_ci" character set!! After I changed them to Chinese character set, everything worked perfectly.
So, to those newbies who would like to use languages other than English (especially Asian languages), DO NOT load sample data while you finish to install VM. However, if you had loaded sample data, then you have to change character sets one by one, no big deal.
(I don't have Joonfish installed.)